The present invention relates to nuclear-based contraband detection systems, and more particularly to an apparatus and method for accurately detecting contraband concealed within a container, such as a suitcase, truck or other object. As used herein, the term "contraband" includes, but is not limited to, explosives and illicit drugs.
Diagnostic nuclear techniques in general involve use of two highly penetrating radiations (e.g., neutrons and gamma rays) which enable one to detect concealed explosives or other contraband materials. The radiations act as follows: An appropriately fashioned primary radiation excites atomic nuclei within a designated volume of an object. The excited atomic nuclei subsequently relax, emitting electromagnetic or particle radiation in the process that is characteristic of the nuclear species. The analysis of the emitted spectrum thus facilitates the detection of a particular substance within the object, e.g., explosives or illegal drugs. In other words, if the emitted spectrum includes radiation of a given energy, then the presence of a particular element within the object can be inferred. Thus, a spectrum showing characteristic radiation lines of particular intensities serves as a "signature" that identifies the presence of a particular chemical element within the object being examined. Identifying the chemical elements and/or chemical compounds within an object thus involves identifying the corresponding signatures that are present in the radiations emitted from the material. See e.g., Gozani, Active Nondestructive Assay of Nuclear Materials, United States Nuclear Regulatory Commission, NUREG-CR-0602, SAI-FM-2585 (1981).
It is common practice to use neutrons as the primary radiation and to measure the ensuing gamma-ray spectra for non-intrusive diagnostic purposes. U.S. Pat. No. 3,832,545 and patent application Ser. No. 07/053,950, filed 05/26/87, for example, disclose nuclear-based explosive detection systems that make use of neutrons of mainly thermal energies. In contrast, European Patent publication EP-O-227-497-A1 discloses a nuclear-based explosive detection system wherein fast neutrons of energies from 7 to 15 million electron volts (MeV) are employed. Disadvantageously, the thermal neutron based detection systems provide, for practical purposes, primarily only one signature of the four cardinal constituents of explosives, namely the signature of nitrogen (and possibly hydrogen). The fast neutron based detection system, on the other hand, may provide signatures of all four ingredients of explosives, or other contraband, thus enhancing the interrogating power of the fast neutron contraband detection systems. (The four cardinal chemical constituents of explosives are hydrogen, carbon, nitrogen, and oxygen.)
It must be observed, however, that simply obtaining the signatures of the constituent elements of a specified contraband does not necessarily indicate that such contraband is present in the object under investigation. This is because many benign materials (non-contraband) also include such elements. A great diagnostic advantage may thus be obtained when a three-dimensional image of the distribution of element densities within the interrogated body is also formed, as such image of densities may help further distinguish contraband from non-contraband. A suitable three-dimensional image for this purpose may advantageously be obtained by performing a section-by-section neutron irradiation of the object, and by performing a computer-based analysis of the energy and intensity of the gamma rays that are produced in each section. Such analysis has in the past required the judicious positioning of gamma-ray detectors around the object, as taught in Applicants' earlier patent application, Ser. No. 07/053,950, filed 05/26/87.
As previously indicated, neutron interrogation of objects for the detection of contraband, e.g., explosives, is known in the art. One of the most common forms of neutron interrogation, and the only form that has yet been commercialized, is thermal neutron activation (TNA). In the TNA techniques, the object being interrogated is exposed to low energy neutrons, causing gamma rays having an energy characteristic of the element(s) within the object to be generated. The gamma rays of a particular energy are detected and counted. From such count, a determination can be made as to the abundance of nitrogen within the object being interrogated. The ability of TNA techniques to reliably detect the explosives depends greatly on the large nitrogen content and density of the explosive.
Another technique known in the art for detecting explosives is fast neutron activation (FNA). FNA techniques are similar to TNA techniques in that an object being interrogated is bombarded with neutrons. However, in the case of FNA, the neutrons have a higher energy, e.g., 14 MeV, and the gamma rays they generate allow the presence of additional elements to be detected. In particular, FNA allows the presence of hydrogen, carbon, and oxygen to be detected in addition to nitrogen. The relative concentrations of all of these elements thus comprise a "signature" that further helps to identify a particular substance, i.e., contraband.
A still further technique for detection of explosives involves detection of the alpha particle generated in a T(d,n).sup.4 He reaction which produces a 14 MeV neutron. The neutron and alpha particle are emitted in opposite directions. A small particle detector near the tritium target detects the alpha particle. The corresponding neutron is emitted at 180.degree. within a solid angle equal to the solid angle subtended by the alpha detector from the target. This solid angle defines a "beam" of neutrons that is used to interrogate a sample, such as a suitcase or other container. A gamma ray detector is placed near the sample, detecting gamma rays in coincidence with the alpha particles. The time difference between the alpha particle detection and gamma ray detection can provide the position of the gamma ray source along the beam. By scanning the beam, a three-dimensional image of the gamma ray sources can thus be generated.
Finally, as indicated in French patent document #EP 0 227 497 A1, and a paper recently presented in the 5th Asia/Pac Aviation Seminar in Kuala Lumpur (Aug. 17-21, 1987), it is also known in the art to combine both fast and thermalized neutrons in the same detection system. As indicated in these documents, a partially moderated 14 MeV pulsed neutron source is used with one or more well shielded germanium detector(s). Nitrogen and oxygen are determined through (n,x.gamma.) reactions during the bursts of the fast neutrons, and hydrogen and chlorine are determined between pulses through (n,.gamma.) reactions with thermalized neutrons.